Motor drive voltage control device and method for controlling motor drive voltage

ABSTRACT

To suppress a decline in the control accuracy of an applied voltage associated with an increase in quantum noise, and to increase the control accuracy of a motor speed. When generating a driving voltage signal supplied to a motor from a driving command signal, a motor-driving voltage control device reduces the gradation level and performs noise-shaping modulation before performing PWM modulation. Reducing the gradation level allows the degree of gradation of the driving voltage signal to be within the resolution range of the PWM modulation, and thus PWM modulation can be performed even when the driving voltage signal has a high frequency. Noise-shaping modulation reduces the level of quantum noise near the low frequency range by causing the quantum noise due to digitization, included in the driving voltage signal, to be biased toward the high frequency range side. Of modulation signals with the reduced-gradation level, the components near the high frequency band are cut, while the components near the low frequency range are used to suppress quantum noise and control the driving voltage applied to the motor with a high accuracy.

CROSS-REFERENCE TO RELATED APPLICATION

The present disclosure is a continuation of U.S. patent application Ser.No. 14/648,289 filed on May 29, 2015, which is a 371 of internationalapplication PCT/JP2013/083313 filed on Dec. 12, 2013, which claimspriority to Japanese patent application 2012-280355 filed on Dec. 22,2012, the entire disclosures of each of which are herein incorporated byreference.

FIELD OF THE DISCLOSURE

The present invention relates to the field of motor-driving and toreducing the quantization noise of the driving voltage provided for amotor.

BACKGROUND OF THE DISCLOSURE

It is known that the rotating speed of a motor is controlled bymodulating the amplitude of the voltage applied to the motor by usingPWM (Pulse Width Modulation). For PWM control, the amplitude of thecontrol signal that controls motor-driving is modulated to the pulsewidth, and applying the driving voltage signal that is modulated usingPWM to the motor allows the power provided to the motor to becontrolled, thereby controlling the rotating speed.

FIG. 4(a) is a schematic view of the motor control using PWM control. Inthe PWM control of the motor shown in FIG. 4(a), the amplitude of adriving command signal that controls the rotation in a PWM modulator 101is modulated to the pulse width using PWM, and then the modulated signalby using pulse width modulation is applied to the motor 20.

It is known that the rotating speed varies as a result of ripple of thedriving voltage in the motor control using PWM control.

When the driving voltage (PWM signal) obtained using PWM modulation isapplied to the motor, a smoothed driving current flows in the motor.Then, the ripple of the driving current of the motor is generated as aresult of the variation of the driving current while the driving currentfollows a change of the driving voltage that is a PWM signal. The rippleof the driving current increases with the conversion of the drivingvoltage. FIGS. 6(a) and (b) explain the ripple that is generated duringPWM modulation. When the amplitude of the driving voltage is small, theripple is small due to the small voltage variation. However, when theamplitude of the driving voltage is large, the ripple increases due tothe large voltage variation. The broken lines in FIGS. 6(a) and (b) showone example of the ripple.

A technology that allows torque ripple to decrease in the motor controlby using a Δ-Σ modulation has been known (Patent document 1).

Patent document 1 discloses a technique that reduces the torque ripplethat appears in an offset due to a switching delay of comparators and/orMOSFETs constituting PWM circuits, by applying a negative feedback ofthe output signals of Δ-Σ modulation at the time of applying the voltageto the coil in the motor-driving by the PWM modulation.

RELATED ART

Patent document 1: JP5-344780 (paragraphs [0007] and [0010]).

Patent document 2: JP2003-124812

Problems to be Solved

In the PWM modulation of the driving command signal using digitalprocessing, the amplitude resolution of the driving command signaldepends on the clock period. It is limited to the width of one clockperiod. Since the width of one clock period is set to a minimum unit,the resolution is unable to be set to be smaller than the width of oneclock period. Thus, there is a problem in that PWM modulated signalsinclude the quantization noise that is generated when the signalamplitude is quantized using PWM modulation. The quantization noise isan amplitude error generated when the signal amplitude is quantized, andit has one half of the quantization step width.

There is a problem in that quantization noise reduces control accuracyin the motor speed control by the PWM control. The decrease of the motorcontrol accuracy due to the quantization noise will be explained belowin reference to FIG. 4(b).

There exist the two following factors, for example, that cause anincrease in the quantization noise. Factor 1: The first factor is thatthe driving command signal has large variations in its range. If theresolution of PWM is fixed, the number of bits that are needed torepresent the driving command signal is limited. Thus, the larger therange of the variation in the voltage of the driving command signal is,the larger the quantization noise is.

Factor 2: The second factor is that the PWM resolution is reduced inaccordance with the increase in the frequency of PWM.

Even if pulse voltage is applied to the motor in PWM control, thecurrent that is smoothed using the inductance of the motor coil flowsthrough the motor, and thus a smooth operation can be obtained. However,when the range of the variation in the pulse voltage is so large thatsmoothing cannot follow the variation, thereby generating the torqueripple, the frequency of PWM control needs to be increased by the stepof increasing the frequency, thereby supressing the range of thevariation in the pulse voltage. The PWM resolution needs to be decreasedin order to increase the frequency of PWM control, because electroniccircuits have a limited speed of processing. Decreasing the resolutionin PWM may cause the quantization noise to increase.

Since, in the PWM modulation, the length of the period of performing themodulation is expressed as a product of a clock period times the numberof clocks within one period (resolution), if the clock period isconstant, and the length of the period of the modulation is shortened byincreasing the frequency, the number of pulses (resolution) within oneperiod decreases. Since the resolution is associated with the gradationnumber and a low resolution causes it to be difficult to set asignificant gradation number, increasing the frequency is a factor inincreasing the quantization noise.

FIG. 5 shows the relationship between the length of the period and theresolution in the PWM modulation. FIGS. 5(a)-(c) show a case in whichthe modulation frequency of the PWM modulation is low. FIGS. 5(d)-(f)show a case in which the modulation frequency of the PWM modulation ishigh.

FIGS. 5(a) and (d) show the length of the period of performing themodulation in PWM modulation. FIGS. 5(b) and (e) show one pulse withinthe length of the period, and FIGS. 5 (c) and (f) show clocks within thelength of the period.

Since the length of the period is shortened by causing the frequency tobe higher (FIGS. 5(a) and (d)), the number of clocks within one perioddecreases if the clock period is constant (FIGS. 5(c) and (f)). Sincethe number of clocks within one period is associated with theresolution, the resolution decreases by causing the frequency to behigher.

Motors used in robots, industrial machines, and other areas that arerequired to have a high output are often required to convert the appliedvoltage to the motors into a high voltage to obtain a high motor output.This conversion into the high voltage further increases the quantizationnoise caused by in the two factors mentioned above.

In the first factor, the conversion of the driving command signal into ahigher voltage increases the range of the voltage variation, therebyincreasing the quantization noise.

In the second factor, the idea of increasing the modulation frequency ofthe PWM modulation comes to mind as a way to reduce the influence on theincrease of the ripple involved with increasing the driving commandvoltage to a significantly high voltage.

FIGS. 6(c) and (d) illustrate reduction of the ripple by causing themodulation frequency to be higher. FIG. 6(c) shows a case where themodulation frequency of the PWM modulation is low, and FIG. 6(d) shows acase where the modulation frequency of the PWM modulation is high.Ripple r_(H) (see FIG. 6(d)), where the modulation frequency is high,became smaller than ripple r_(L) (FIG. 6(c)), where the modulationfrequency is low, because the length of the period (one period width),where the modulation frequency is high, is shorter than that where themodulation frequency is low.

The reduction of the ripple by causing the modulation frequency to behigher will further heighten the quantization noise increase due to thesecond factor.

The quantization noise increase will reduce the control accuracy of thevoltage applied to the motor, and further reduce the control accuracy ofthe motor speed.

Thus, the objects of the present invention include solving the problemsof the conventional techniques explained above, suppressing thereduction of the control accuracy of the applied voltage due to thequantization noise increase, and increasing the control accuracy of themotor speed.

SUMMARY OF THE INVENTION

The present invention has a feature of lowering a gradation level andperforming noise-shaping modulation before performing PWM modulation,when the driving voltage signal to be supplied to the motor is generatedfrom the driving command signal. Lowering a gradation level allows thegradation level of the driving voltage signal to reach such a level thatit falls within the resolution range of PWM modulation, thereby enablingPWM modulation even though the driving voltage signal has a highfrequency.

The noise-shaping modulation allows the quantization noise caused bydigitization that is included in the driving voltage signal to be biasedtoward the high-frequency range, thereby reducing the level of thequantization noise in the low-frequency range side. Within the frequencyrange of the modulated signal where the gradation level is lowered, thecomponents of the high-frequency range are cut, and the components ofthe low-frequency range side are used, thereby suppressing thequantization noise. The driving voltage applied to the motor iscontrolled with a high degree of accuracy by suppressing thequantization noise. This suppressing effect on quantization noise can bemore significant when the driving voltage signal is converted into ahigh voltage.

Since the quantization noise biased toward high-frequency range side bynoise-shaping modulation can be cut off by the low-pass filter havingthe inductance of a coil (s), which the motor includes, the coil whichthe motor includes can be used as not only an element of the motoritself but also an element of the low-pass filter. The noise-shapingmodulation can employ Δ-Σ modulation, for example. Also, it can employanother modulation system as well (for example, see patent document 2).

The present inventions include two aspects, such as a motor-drivingvoltage control device and a motor-driving voltage control method.

One aspect of the motor-driving voltage control device of the presentinvention includes a motor-driving voltage control device that generatesa driving voltage signal to be provided for a motor based on a drivingcommand signal, comprising the following: a low gradation unit thatlowers the gradation level of the driving voltage signal and performsnoise-shaping modulation in which quantization noise is biased toward ahigh-frequency range, and a PWM modulator that modulates the amplitudeof the driving voltage signal to the pulse width thereof, the drivingvoltage signal having the low gradation level generated in the lowgradation unit.

The low gradation unit causes the gradation level of the driving voltagesignal to be lower to such a level that it falls within the resolutionrange of the PWM modulator, and causes the frequency components of thequantization noise to be unevenly distributed in the frequency sidehaving higher frequencies than the cutoff frequency of the low-passfilter that includes inductance of the motor by the noise-shapingmodulation.

The PWM modulator supplies to the motor a driving voltage signal, thelow gradation unit causing the gradation level of the driving voltagesignal to be lower and causing the frequency components of thequantization noise thereof to be unevenly distributed in the higherfrequency side.

The quantization noise that is biased toward the high-frequency rangeside of the driving voltage signal is cut by the low-pass filter thatincludes the inductance of the motor, thereby reducing the quantizationnoise of the driving voltage signal to be supplied to the motor.

The motor-driving voltage control device of the present invention may beconfigured to comprise a high voltage converter that converts thevoltage of the driving voltage signal generated in the PWM modulatorinto a high voltage signal. The driving voltage signal, the voltage ofwhich is increased in the high voltage converter, is supplied to themotor, thereby allowing the output of the motor to increase.

The motor-driving voltage control device of the present invention isconfigured to comprise a high frequency converter that increases thefrequency of the driving command signal in the preceding stage of thelow gradation unit. The high frequency converter may increase thefrequency of the driving command signal, thereby allowing smoothing bythe low-pass filter.

The high frequency converter of the present invention is configured tooversample the driving command signal at a higher frequency than thecutoff frequency of the low-pass filter.

Generally, in the driving control of the PWM modulation, frequencycomponents of the driving command signal having higher frequencies thanthe cutoff frequency of the low-pass filter are blocked in the low-passfilter. Thus, when the digitized driving command signal is set on thefrequency, it is sufficient to set frequency components which have alower frequency than the cutoff frequency of the low-pass filter, andwhich allows a necessary degree of accuracy to be obtained.

However, when the driving command signal is received at a lowerfrequency than the cutoff frequency of the low-pass filter, even thoughnoise-shaping is performed with the driving command signal, thequantization noise cannot be unevenly distributed in the frequency sidehaving the higher frequencies than the cutoff frequency. Thus, no effecton reducing the quantization noise takes place.

Thus, if the high-frequency components in the frequency domain of thedriving command signal are set to have a lower frequency than the cutofffrequency of the low-pass filter, the frequency components having higherfrequencies than the cutoff frequency of the low-pass filter areobtained by oversampling the driving command signal, namely by sampling,at a high-frequency, the frequency components of the driving commandsignal having the lower frequency than the cutoff frequency of thelow-pass filter. Thereafter, the quantization noise of the obtaineddriving voltage signal is caused to be unevenly distributed in thehigher frequency than the cutoff frequency of the low-pass filter byperforming the noise-shaping modulation, thereby lowering the gradationlevel. Thereby, the noise that is biased toward the high-frequencydomain is cut off by the low-pass filter.

However, when the driving command signal is sampled at a high frequencyand the high-frequency components of the frequency domain of the drivingcommand signal are set to be at a higher frequency than the cutofffrequency of the low-pass filter, oversampling need not be performed.Thus, the quantization noise is caused to be unevenly distributed in afrequency range having higher frequencies than the cutoff frequency ofthe low-pass filter, by directly performing noise-shaping modulationwithout oversampling the driving command signal, and the gradation levelis lowered, thereby removing the noise that is unevenly distributed inthe high-frequency range by the low-pass filter.

The low gradation unit of the present invention can use a Δ-Σ modulatoras a configuration that causes the gradation level to be lower andperforms the noise-shaping modulation. Other types of modulation systemsin addition to the Δ-Σ modulation can be used for the noise-shapingmodulation.

The Δ-Σ modulator can perform the noise-shaping modulation using aconfiguration where the frequency components of the quantization noiseare biased toward the high-frequency side by using a configuration ofapplying feedback of an output obtained through an integrator to aninput, and can lower the gradation level by a quantizer that quantizes amodulated signal.

One aspect of the motor-driving voltage control method of the presentinvention includes the motor-driving voltage control method forgenerating a driving voltage signal to be supplied to a motor based on adriving command signal, comprising the following: lowering the gradationlevel of the driving voltage signal and performing noise-shapingmodulation in which quantization noise is biased toward a high-frequencyrange, and modulating the amplitude of the driving command signal to thepulse width thereof, the driving command signal having a low gradationlevel generated in lowering the gradation level and performing thenoise-shaping modulation.

Lowering the gradation level and performing the noise-shaping modulation(a low gradation process) include the following: causing the gradationlevel of the driving voltage signal to be lower to such a level that itfalls within the resolution range of the PWM modulator, and causing thefrequency of the quantization noise to be unevenly distributed in thefrequency side having higher frequencies than the cutoff frequency of alow-pass filter that includes the inductance of the motor using thenoise-shaping modulation.

The modulating amplitude of the driving voltage signal to pulse widththereof (a PWM modulation process) includes supplying to the motor adriving voltage signal, the gradation level of the driving voltagesignal being caused to be lower and the frequency components of thequantization noise being caused to be unevenly distributed in a higherfrequency side by lowering the gradation level and performing thenoise-shaping modulation. The driving voltage signal obtained by the PWMmodulation process passes through the inductance of the motor. Then thequantization noise that is unevenly distributed to the high-frequencyside of the driving voltage signal is cut off by the low-pass filterhaving the inductance, thereby reducing the quantization noise of thedriving voltage signal supplied to the motor.

The motor-driving voltage control method of the present inventioncomprises converting a voltage of the driving voltage signal generatedin the PWM modulation process into a high voltage signal (a high voltageconversion process).

The motor-driving voltage control method of the present inventioncomprises increasing the frequency of the driving command signal (ahigh-frequency converting process) in the preceding process of the lowgradation process. The high-frequency process includes increasing thefrequency of the driving command signal to a high-frequency wheresmoothing of the signal is made possible by the low-pass filter.

The high-frequency converting process of the present invention includesoversampling the driving command signal at a higher frequency than acutoff frequency of the low-pass filter in the preceding process of thelow gradation process.

When the high-frequency range of the frequency domain of the drivingcommand signal is set to a lower frequency than the cutoff frequency ofthe low-pass filter, this process includes sampling the driving commandsignal having lower frequencies than the cutoff frequency of thelow-pass filter at a high-frequency by oversampling. Thereby thefrequency components having higher frequencies than the cutoff frequencyof the low-pass filter are obtained.

When the driving command signal is received at a lower frequency thanthe cutoff frequency of the low-pass filter, the quantization noisecannot be unevenly distributed in a frequency side having higherfrequencies than the cutoff frequency even if the noise-shaping of thedriving command signal is performed. Thus, the effect on the reductionof the quantization noise disappears. This is a problem. However, thisproblem can be resolved by increasing the frequency of the drivingcommand signal by using oversampling.

When the driving command signal is sampled at a high frequency and thehigh-frequency range of the frequency domain of the driving commandsignal is set to be a higher frequency than the cutoff frequency of thelow-pass filter, there is no need to increase the frequency by usingoversampling, etc.; thus the gradation level of the driving commandsignal is directly caused to be lower in the low gradation processwithout performing the high-frequency converting process.

The high-frequency converting process extends the frequency range wherethe quantization noise is distributed. Since the quantization noise isevenly distributed in the frequency range, lowering the gradation levelof the driving command signal after increasing the frequency of thedriving command signal using oversampling can cause the quantizationnoise level in the low-frequency domain to be lower than that of thedriving command signal that is sampled at a normal sampling rate.

By the motor-driving voltage control device and the control method ofthe present invention, when the motor-driving voltage is controlled togenerate the driving voltage signal that is supplied to the motor basedon the driving command signal, a gradation signal of the driving voltagesignal, the frequency of which is caused to be high, may be generated bysampling the driving command signal at a high-frequency. The gradationof the PWM signal needs to be decreased and the frequency needs to bemade high to decrease the ripple width and reduce the quantizationnoise. Then, the frequency of the driving voltage signal can be causedto be as high as the frequency of the PWM modulation to be used even ifthe frequency of the original driving voltage signal is lower than thefrequency of the PWM, by generating the driving voltage signal where thegradation level is set by using a high frequency.

When the gradation level of the driving command signal obtained from thecontroller, etc., is set by using a high frequency from the verybeginning, it is not necessary to cause the frequency of the drivingcommand signal to be high by using sampling, etc.

The gradation level of the gradation signal of the driving commandsignal is caused to be lower before modulating using PWM. In themodulated signal, the gradation level of which is lowered, thequantization noise is unevenly distributed in the high-frequency side bythe noise-shaping modulation in the step of lowering the gradationlevel; thus the quantization noise level in the low-frequency side isreduced. Furthermore, when the gradation level is lowered, the gradationlevel of the modulated signal is reduced, and the driving voltage signalhaving a lower gradation level than that of the driving command signalis generated. Lowering the gradation level allows the PWM modulationcircuit to cope, from the standpoint of the processing ability, with thePWM modulation of the driving command signal, the frequency of whichincreases.

After the PWM modulation of the driving command signal, the gradationlevel of which is lowered, and the generation of the driving voltagesignal, the signal passes through the low-pass filter, therebyeliminating the quantization noise biased toward the high-frequency sideby using the noise-shaping of lowering a gradation level, and thequantization noise of the driving voltage signal to be supplied to themotor is distributed only for the low-frequency side. Since thequantization noise in the low-frequency side is reduced by thenoise-shaping in the step of lowering the gradation level, thequantization noise applied in the motor is reduced. The inductance ofthe motor can be utilized as a component of the low-pass filter.

By the aspect of the present invention, when the frequency of the PWMmodulated signal is caused to be increased to suppress ripple, thequantization noise increase that is accompanied by the increase of thefrequency can be suppressed, thereby allowing the voltage applied to themotor to be controlled with a high degree of accuracy.

EFFECT OF THE INVENTION

As explained above, the motor-driving voltage control device and themotor-driving voltage control method of the present invention cansuppress the decrease of the control accuracy of the applied voltageaccompanied by quantization noise increase, and can increase the controlaccuracy of the motor speed when converting the voltage of the signalused in the motor control into a high voltage.

BRIEF EXPLANATION OF FIGURES

FIG. 1 is a block diagram of the motor-driving voltage control system ofthe present invention.

FIG. 2 is a diagram that shows the quantization noise of each section ofthe motor-driving voltage control system.

FIG. 3 is a diagram that shows an exemplary circuit configuration of Δ-Σmodulation of the motor-driving voltage control.

FIG. 4 is a schematic view of the motor control using the PWM controland a diagram that shows the quantization noise increase.

FIG. 5 is a diagram that shows the relation between the gradation numberof the signal and the quantization step width and the relation betweenthe gradation number of the signal and the quantization noise.

FIG. 6 is a diagram that shows ripple generated using PWM control.

DETAILED DESCRIPTION OF THE INVENTION

The embodiments of the present invention will be explained in detailbelow with reference to figures. Configuration examples of themotor-driving voltage control device of the present invention will beexplained below with reference to FIGS. 1, 2, and 3.

FIGS. 1, 2, and 3 illustrate configuration examples of the motor-drivingvoltage control system of the present invention. FIG. 1 illustrates aschematic view. FIG. 2 illustrates the quantization noise of eachsection. FIG. 3 illustrates an example of a Δ-Σ modulation circuit.

Configuration Examples of the Present Invention

The configuration examples of the motor-driving voltage control systemof the present invention will now be described. FIG. 1(a) illustrates aconfiguration example where the driving command signal input from acontroller, etc., has a frequency lower than or equal to the cutofffrequency of the low-pass filter of the motor side. FIG. 1(b)illustrates a configuration example where the driving command signalinput from the controller, etc. has a frequency higher than the cutofffrequency of the low-pass filter of the motor side.

As shown in FIG. 1(a), a motor-driving voltage controller 1 includes ahigh frequency converter 3 that increases the frequency of the drivingcommand signal received from the controller, a low gradation unit 4 thatcauses the gradation level of the driving command signal having thehigh-frequency to be lower and performs noise-shaping, and a PWMmodulator 5 that modulates the amplitude of the output of the lowgradation unit 4 to pulse width to generate the driving voltage signal.Applying the driving voltage signal generated in the PWM modulator 5 tothe motor 20 leads to performing the driving control based on thedriving command signal.

A high voltage converter 6 may be connected to the PWM modulator 5. Theoutput of the motor can be caused to be high by applying the drivingvoltage signal to the motor 20. the driving voltage signal having beenconverted into a high voltage signal in the high voltage converter 6.

The motor-driving voltage control device 1 in FIG. 1(b) does not includethe high frequency converter 3 that is disposed between the controller 2and the low gradation unit 4 in FIG. 1(a). Everything that does notinclude the frequency converter 3 is the same as the configuration inFIG. 1(a). The high frequency converter 3 is configured to include acircuit that performs oversampling, i.e., samples the signal at a higherfrequency than the cutoff frequency of the low-pass filter.

Since the inductance of the motor acts as a low-pass filter, thefrequency components of the driving voltage signal having higherfrequencies than the cutoff frequency of the low-pass filter are cut offSo they do not work as the motor-driving. Thus, the driving commandsignal input from the controller or the like is sufficient, as long asit has the frequency characteristics that allow the motor to becontrolled with the required degree of accuracy at a lower frequencyrange than the cutoff frequency of the low-pass filter.

However, when the driving command signal is sent at a lower frequencythan the cutoff frequency of the low-pass filter, the quantization noisecannot be unevenly distributed in a higher frequency side than thecutoff frequency even if the noise-shaping of the driving command signalis performed. So there is no effect on reducing the quantization noise.

Thus, when the frequency of the driving command signal is set to belower than the cutoff frequency of the low-pass filter, the frequency ofthe driving command signal is caused to be higher than the cutofffrequency of the low-pass filter by oversampling it at a high-frequency.After that, the quantization noise is biased toward the high-frequencyrange by using the noise-shaping modulation and the gradation level islowered.

FIG. 1(a) shows a configuration that is applied when the high-frequencyrange of the frequency domain of the driving command signal is set to belower than the cutoff frequency of the low-pass filter. The highfrequency converter 3 increases the frequency of the driving commandsignal to a higher frequency than the cutoff frequency of the low-passfilter.

However, when the driving command signal has a higher frequency than thecutoff frequency of the low-pass filter that is formed using theinductance of the motor, it is not necessary to increase a frequencyusing oversampling. Thus, the high frequency converter 3 can be omitted.FIG. 1(b) shows a configuration that is applied when the frequency ofthe driving command signal is set to be higher than the cutoff frequencyof the low-pass filter.

The high frequency converter 3 samples the driving command signal at ahigh-frequency in the preceding stage of the low gradation unit 4, andlowers the gradation level at the high-frequency to generate the drivingcommand signal, the frequency of which is caused to be high. The drivingcommand signal is input from the controller 2. The driving commandsignal may drive a motor that drives a driving mechanism for industrialmachines and arms and legs for a robot, but is not limited to theseapplications.

Increasing the frequency of the driving command signal can be done byshortening the clock interval between one timing and next timing forquantizing the signal amplitude. Shortening the period width of pulsesof the PWM signal reduces the ripple of the driving current whiledriving the motor. A frequency close to the PWM frequency can be used asa target frequency for increasing the frequency.

The high frequency converter 3 can be realized by using an oversamplingcircuit, for example. When the high-frequency components are set to alower frequency than the cutoff frequency of the low-pass filter of themotor 20 in the frequency range of the driving command signal from thecontroller 2, the high frequency converter 3 oversamples the drivingcommand signal at a high-frequency, and a signal having higherfrequencies than the cutoff frequency of the low-pass filter isobtained.

When the driving command signal is sent from the controller at a lowerfrequency than the cutoff frequency of the low-pass filter, even if thenoise-shaping of the driving command signal is performed, thequantization noise cannot be caused to be unevenly distributed in thefrequency side having higher frequencies than the cutoff frequency. Sothe problem where there is no quantization noise-reduction effectarises. To resolve this problem, the frequency of the driving commandsignal is caused to increase to a high-frequency in the high frequencyconverter 3. So a higher frequency is obtained than the cutoff frequencyof the low-pass filter. Thereby the noise can be unevenly distributed inthe frequency domain having higher frequencies than the cutoff frequencyof the low-pass filter in noise-shaping. Thus the noise unevenlydistributed in the high-frequency domain can be cut off in the low-passfilter of the motor.

In addition, when the driving command signal from the controller issampled at a higher frequency than the cutoff frequency of the low-passfilter, it is not necessary to increase the frequency by usingoversampling in the high frequency converter 3, etc. In a case likethis, it is not necessary to increase the frequency, and the gradationlevel of the driving command signal is lowered by using thenoise-shaping modulation in the low gradation unit 4 without increasingthe frequency.

Thus, the high frequency converter 3 can be selectively arranged basedon the frequency of the driving command signal received from thecontroller 2. Namely, when the frequency of the driving command signalis set to be lower than the cutoff frequency of the low-pass filter, thehigh frequency converter 3 is arranged in the preceding stage of the lowgradation unit 4. When the frequency of the driving command signal isset to be higher than the cutoff frequency of the low-pass filter, thelow gradation unit 4 need not be arranged.

Also, when the high frequency converter 3 increases the frequency, thequantization noise can be reduced most efficiently by oversampling atthe carrier frequency of PWM.

The low gradation unit 4 lowers the gradation level of the drivingcommand signal, the frequency of which is caused to increase in the highfrequency converter 3. The low gradation unit 4 causes the gradationlevel of the driving voltage signal to be lower to such a level that itfalls within the resolution range of the PWM modulator 5, and causes thefrequency components of the quantization noise to be unevenlydistributed in a higher frequency side than a cutoff frequency of alow-pass filter, which includes the inductance of the motor by thenoise-shaping modulation. For example, the low gradation unit 4 canperform the above operation using a Δ-Σ modulation. The quantizationnoise generated due to the digital processing of the high frequencyconverter 3, etc., is distributed uniformly within a frequency range ofone half of the sampling frequency.

The noise-shaping modulation in the low gradation unit 4 causes thequantization noise to be biased toward the high-frequency range side,and reduces the quantization noise level of the low-frequency rangeside. Since the generated quantization noise energy is constant, anabsolute amount of the quantization noise level in the low-frequencyrange side is reduced by using a distribution profile where thequantization noise is biased toward the high-frequency range and thequantization noise level in the low-frequency range side is reduced.

The gradation number of a modulated signal of the low gradation unit 4is decreased by lowering the gradation level. So the driving voltagesignal with a lower gradation level than the driving command signaltransmitted from the controller 2 is generated.

The driving command signal, the gradation level of which is lowered inthe low gradation unit 4, and where the noise-shaping modulation isperformed, is modulated using the PWM in PWM modulator 5. It may beconverted into the high voltage signal in the high voltage converter 6.After that the signal may be applied to the motor 20. Alternatively itmay be directly applied to the motor 20 without passing through the highvoltage converter 6.

The PWM modulator 5 supplies to the motor 20 the driving voltage signal.The gradation level of the driving voltage signal is lowered in the lowgradation unit 4 and the quantization noise is biased toward thehigh-frequency side.

The inductance of coils that are included in the motor 20 makes up thelow-pass filter 21, and it cuts off the high-frequency components of thedriving voltage signal that is applied, leaving only low-frequencycomponents as the quantization noise.

The high voltage converter 6 converts the voltage of the driving voltagesignal output from the PWM modulator 5 into a high voltage signal.Increasing the voltage of the driving voltage signal expands the rangein which the driving voltage signal can be set and allows themotor-driving to be controlled with a high degree of accuracy.

Next, the quantization noise of each part is explained. FIG. 2(a) showsthe quantization noise of the driving command signal and thequantization noise of the driving command signal after the frequency isincreased. FIG. 2(b) shows the quantization noise of the driving commandsignal after the noise-shaping modulation. FIG. 2(c) shows thequantization noise of the driving command signal after the signal passesthrough the low-pass filter.

In FIG. 2(a), the quantization noise of the driving command signal(shown by a broken line in the Figure) is expanded into thehigh-frequency range and the level decreases by increasing the frequencyusing oversampling, etc., as shown in the portion of a solid line andoblique lines in the Figure. Furthermore, in the stage of increasing thefrequency of the driving command signal, the frequency level of thequantization noise is uniform in terms of frequency.

Performing the noise-shaping modulation in FIG. 2(b) allows thefrequency components of the quantization noise to be biased toward thehigh-frequency range side. The broken line in the Figure indicates thequantization noise of the driving command signal which is biased. Thearea where a solid line and oblique lines are drawn in the Figureindicates the quantization noise of the driving command signal, thefrequency of which is increased. The noise-shaping modulation allows thelevel of the quantization noise in the high-frequency range side to behigh and allows the level of the quantization noise in the low-frequencyrange side to be low. The distribution of the quantization noise shownin FIG. 2(b) varies linearly. However, this distribution is just oneexample, and the distribution profile where the frequency components ofthe quantization noise are biased toward the high-frequency side variesdepending on the characteristics of the circuit performing Δ-Σmodulation.

FIG. 2(c) shows the quantization noise of the driving voltage signalwhich has passed through the low-pass filter. The quantization noise inthe higher frequency range than the cutoff frequency of the low-passfilter is cut off, and only the quantization noise in a frequency rangehaving lower frequencies than the cutoff frequency passes through thelow-pass filter.

The broken line in the Figure shows the quantization noise that isbiased, of the driving command signal. The area where a solid line andoblique lines in the Figure are drawn shows the quantization noise ofthe driving command signal, the frequency of which is increased.

The quantization noise transmitted to the motor is further reduced byincreasing the frequency of the driving command signal and performingthe noise-shaping modulation, compared with a case where the frequencyof the driving command signal is not increased or the noise-shapingmodulation is not performed.

FIG. 3 shows an example of a circuit where the low gradation unit isconfigured employing Δ-Σ modulation. The circuit employing Δ-Σmodulation in FIG. 3 is a configuration example of a second-order errorfeedback type. The configuration example in FIG. 3 can be configured byadders, delay units, coefficient units, and a quantizer. The output ofthe quantizer which is quantized by the quantizer and whose gradationlevel is lowered is fed back to the input side.

The present invention is not intended to be limited to the aboveembodiments. It is possible to modify the above embodiments in variousways based on the spirit of the present invention, and thesemodifications are not excluded from the scope of the invention.

INDUSTRIAL APPLICABILITY

The motor-driving control device and the method for controlling themotor drive according to the present disclosure can be applied not onlyto the motors provided for driving mechanisms of robots, but also to themotors provided for driving mechanisms of industrial machinery.

DESCRIPTION OF THE REFERENCE NUMERALS

-   1 motor-driving voltage control device-   2 controller-   3 high frequency converter (over-sampling unit)-   4 low gradation unit-   5 modulator-   6 high voltage converter-   20 motor-   21 low-pass filter-   101 modulator

What we claim is:
 1. A motor-driving voltage control device thatgenerates a driving voltage signal to be provided to a motor based on adriving command signal, comprising: a low gradation unit that lowers agradation level of the driving voltage signal and performs noise-shapingmodulation in which quantization noise is biased toward a high-frequencyrange, and a PWM modulator that modulates amplitude of the drivingvoltage signal to pulse width thereof, the driving voltage signal havinga low gradation level generated in the low gradation unit, wherein thelow gradation unit: causes the gradation level of the driving voltagesignal to be lower to such a level that falls within a resolution rangeof the PWM modulator.
 2. The motor-driving voltage control deviceaccording to claim 1, wherein the low gradation unit: causes frequencycomponents of quantization noise to be unevenly distributed in afrequency side having higher frequencies than a cutoff frequency of alow-pass filter that includes inductance of the motor by noise-shapingmodulation.
 3. The motor-driving voltage control device according toclaim 1, wherein the PWM modulator supplies to the motor the drivingvoltage signal.
 4. The motor-driving voltage control device according toclaim 1, wherein the low gradation unit causes the gradation level ofthe driving voltage signal to be lower and causes frequency componentsof quantization noise thereof to be unevenly distributed.
 5. Themotor-driving voltage control device according to claim 1, furthercomprising a high voltage converter that converts the voltage of thedriving voltage signal generated in the PWM modulator into a highvoltage signal.
 6. The motor-driving voltage control device according toclaim 1, further comprising a high frequency converter that increases afrequency of the driving command signal in a preceding stage of the lowgradation unit.
 7. The motor-driving voltage control device according toclaim 6, wherein the high frequency converter increases a frequency ofthe driving command signal to a high-frequency that allows smoothing bya low-pass filter.
 8. The motor-driving voltage control device accordingto claim 6, wherein the high frequency converter oversamples the drivingcommand signal at a frequency higher than a cutoff frequency of alow-pass filter.
 9. The motor-driving voltage control device accordingto claim 1, wherein the low gradation unit comprises a Δ-Σ modulator.10. The motor-driving voltage control device according to claim 9,wherein the Δ-Σ modulator uses second-order error feedback.
 11. Amotor-driving voltage control method for generating a driving voltagesignal to be provided for a motor based on a driving command signal,comprising: lowering a gradation level of the driving voltage signal andperforming noise-shaping modulation in which quantization noise isbiased toward a high-frequency range; and modulating an amplitude of thedriving voltage signal to pulse width thereof, the driving voltagesignal having a low gradation level generated during the lowering of thegradation level and performing the noise-shaping modulation, wherein thelowering the gradation level and performing the noise-shaping modulationincludes: causing the gradation level of the driving voltage signal tobe lower to such a level that falls within a resolution range of a PWMmodulator.
 12. The method for generating a driving voltage signalaccording to claim 11, wherein the lowering the gradation level andperforming the noise-shaping modulation includes: causing a frequency ofquantization noise to be unevenly distributed in a frequency side havinghigher frequencies than a cutoff frequency of a low-pass filter thatincludes inductance of the motor by the noise-shaping modulation. 13.The method for generating a driving voltage signal according to claim11, wherein modulating the amplitude of the driving voltage signal topulse width thereof includes supplying to the motor a driving voltagesignal.
 14. The method for generating a driving voltage signal accordingto claim 11, wherein the gradation level of the driving voltage signalis caused to be lower and frequency components of quantization noise arecaused to be unevenly distributed in a higher frequency side.
 15. Themethod for generating a driving voltage signal according to claim 11,further comprising converting the voltage of a driving voltage signalgenerated in the modulating amplitude of the driving voltage signal intoa high voltage signal.
 16. The method for generating a driving voltagesignal according to claim 11, further comprising increasing a frequencyof the driving command signal in a preceding step of the lowering thegradation level and performing the noise-shaping modulation.
 17. Themethod for generating a driving voltage signal according to claim 16,wherein the increasing the frequency of the driving command signalincluding increasing a frequency of the driving command signal to ahigh-frequency that allows smoothing by the low-pass filter.
 18. Themethod for generating a driving voltage signal according to claim 16,wherein the increasing the frequency of the driving command signalincludes oversampling the driving command signal at a higher frequencythan a cutoff frequency of the low-pass filter.
 19. The method forgenerating a driving voltage signal according to claim 11, wherein thelowering the gradation level and performing the noise-shaping modulationis performed by a Δ-Σ modulator.
 20. The method for generating a drivingvoltage signal according to claim 19, wherein the Δ-Σ modulator usessecond-order error feedback.